1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust pipe of an automotive engine to measure a given component of exhaust emissions, and more particularly to an improved structure of a protective cover of such a gas sensor which is designed to minimize the degree to which a sensing element of the gas sensor is wet with water contained in gases to be measured.
2. Background Art
There are known gas sensors such as oxygen sensors, air-fuel ratio sensors, NOx sensors, or HC sensors which are installed in an exhaust pipe of an automotive engine to measure a given component of exhaust emissions for use in engine burning control or determining the degree of deterioration of a catalyst of a catalytic converter.
Japanese Patent First Publication No. 9-210954 (corresponding to U.S. Pat. No. 5,762,771) teaches a typical one of the above type of gas sensors which is illustrated in FIG. 24.
The gas sensor 9 includes a hollow cylindrical housing 30, a sensing element 35 retained within the housing 30, and a protective cover assembly 9 joined to an end of the housing 30 to cover a sensing portion (i.e., a head portion) of the sensing element 35. The cover assembly 9 has formed in side walls a plurality of holes 910 and 920 through which exhaust gas of the engine flow inside or outside the cover assembly 9.
In order to avoid breakage of the sensing element 35 arising from wetting with water contained in the gas, the cover assembly 9 is made up of an inner cover 92 and an outer cover 91 and so designed that the gas holes 910 formed in the side wall 911 of the outer cover 91 may face the side wall 921 of the inner cover 92 completely without overlapping with the gas holes 920, thereby causing drops of water contained in the exhaust gas entering at the gas holes 910 of the outer cover 91 to hit on and stop at the side wall 921 of the inner cover 92 so that they fall downward and go out of a gas hole 930 formed in a bottom 93 of the outer cover 91.
The above structure, however, encounters the drawback in that the gas holes 910 of the outer cover 91 are spatially blocked by the side wall 921 of the inner cover 92, which results in a difficulty in entrance of the exhaust gas flowing inside the outer cover 91 from the gas holes 910 into the gas holes 920 of the inner cover 92, thus leading to a lowered response rate of the gas sensor 9.
Specifically, the exhaust gasses entering at the gas holes 910 of the outer cover 91, as indicated by arrows G in the drawing, hits against the side wall 921 of the inner cover 92 and rises upward along different paths, thus consuming much time until the exhaust gas enters the inner cover 92 at the gas holes 920 and reaches the sensing element 35.
Additionally, a portion of the exhaust gas entering at one of the gas holes 910, as indicated by one of the arrows G, passes between the bottoms 94 and 93 of the inner and outer covers 91 and 92 and escapes from another of the gas holes 910.